Motor Starting Studies

Motor Starting Studies are performed on a power system to determine unusual consequences of starting a large motor. There are many considerations to starting a motor other than effectively connecting it to the line voltage. Nuisance tripping and excessive running currents, as well as dimming of lights, are signs that a power system isn’t performing properly.

During starting, an AC induction motor or AC synchronous motor will draw greater-than-normal running current, typically about 600% of rated full-load current and will last as long, although diminishing in magnitude, as the motor goes to full speed. If a motor is started with a mechanical load connected to the shaft, inrush current will be drawn for a longer period of time. However, it will not be greater in magnitude than if the motor was started with no load.

The power system should be able to supply inrush to any motor on the system while supplying normal service for the rest of the system. If the system does not have sufficient capacity, there will be excessively low voltage drops and insufficient capacity for motor starting.